PROJECT SUMMARY Immunological memory is the ability of our immune system to respond with greater strength and quickness upon re-encounter with the same pathogen (i.e. secondary infection). Immunological memory is the basis for vaccination which remains the most successful method for preventing infectious disease. Yet, a fully protective vaccine that prevents a single human parasitic disease has not been realized to date. Why is immunity to parasitic pathogens so difficulty to achieve? Our current work on secondary infections with the apicomplexan parasite, Toxoplasma gondii, suggest that failure of immunological memory responses is genetically determined. In this grant submission, we show unpublished data demonstrating the utility of using recombinant inbred panels to discover new requirements for immunity to T. gondii. However, which genes and immune responses are required for immunity against highly virulent strains of T. gondii are not fully understood. We hypothesize the collaborative cross will reveal novel genes previously unknown to be required for immunity to challenge with virulent strains of T. gondii. From published and unpublished work in this system, we predict novel alleles segregating within the CC panel will promote CD8 T cell IFN?- responses during secondary infection and the generation of class switched antibodies with superior capacity to recognize T. gondii. Experimental approaches from immunology, genetics and molecular parasitology will be utilized to test these hypotheses. In Aim 1, the CC panel will be screened to identify loci that determine immunity to virulent challenge, and whether identical loci control protective T and/or B cell immune responses during challenge. In Aim 2, a combination of genetic and transcriptomic approaches will be implemented to confirm and characterize how novel genes and alleles within the CC impact immunity to T. gondii. With the overarching goal of preventing human toxoplasmosis, insights gained from this R21 proposal will guide future projects on vaccination protocols that best prevent toxoplasmosis in susceptible mice.